Most presently-constructed rotorcraft are fitted with one or two free-turbine engines. Power is then taken from a low-pressure stage of the free turbine, which stage is mechanically independent from the compressor assembly and the high-pressure stage of the turbine engine. The free turbine of a turbine engine generally revolves at 20,000 revolutions per minute (rpm) to 50,000 rpm, so it is necessary to have a speed reduction gearbox in the connection with the main rotor of the rotorcraft since its speed of rotation lies substantially in the range 200 rpm to 400 rpm: this is the main transmission gearbox.
The thermal limitations of a turbine engine, and the torque limitations of a main transmission gearbox serve to define a performance envelope covering two normal operational ratings for use of a turbine engine fitted to a rotorcraft having one or two engines:                take-off rating corresponding to a level of torque for the transmission gearbox and a level of heating for the turbine engine that can be accepted for a limited length of time without significant degradation: this is maximum take-off power (PMD) and it can be used for five minutes;        maximum continuous rating during which the capabilities of the transmission gearbox and the capabilities that result from maximum acceptable continuous heating in front of the high pressure blades of the first state of the turbine are not exceeded at any time: this is maximum continuous power (PMC), it can be used without any time limit, and it corresponds to about 90% of PMD.        
On a two-engine rotorcraft, the performance envelope also covers emergency supercontingency ratings, that are used only in the event of one engine being inoperative (OEI):                the supercontingency rating during which the capabilities of the transmission gearbox on the inlet stages and the thermal capabilities of the turbine engine are used to the maximum: this is referred to as super-emergency power (PSU) equal to about 112% to 120% of PMD and it can be used for a maximum of thirty consecutive seconds, and only three times in any one flight, and if ever PSU is used, it is then necessary to remove and overhaul the turbine engine;        a supercontingency rating during which the capabilities of the transmission gearbox on the inlet stages and the capabilities of the turbine engine are used to a very great extent: the power level is then about 105% to 110% of PMD and this level can be used for a maximum of two consecutive minutes, being known as maximum emergency power (PMU); and        the supercontingency rating during which the capabilities of the transmission gearbox on the inlet stages and the thermal capabilities of the turbine engine are used without damage: this level is equal to PMD and can be used continuously for the remainder of a flight after a turbine engine has failed, and is referred to as intermdediate emergency power (PIU).        
Consequently, the thermal and mechanical constraints and also the phenomenon of turbine blade creep lead to the turbine engine being degraded to a greater or lesser extent depending on the rating used. To guarantee safety in flight and to guarantee that performance is achieved, it is therefore essential to determine the maximum amount of damage that a turbine engine can accept.
Thereafter the overall utilization potential of the turbine engine is evaluated. Concretely this reduces to defining a maximum number of flying hours, known as time between overhauls (TBO), that the turbine engine is capable of performing since its most recent overhaul or since its first use, depending on current circumstances. Once this TBO has been reached, the turbine engine is removed and then overhauled.
Below in this text, and for convenience, the term “most recent overhaul of the turbine engine” is used to cover either the first use of the turbine engine or indeed the most recent overhaul thereof.
Furthermore, in order for a rotorcraft to obtain authorization to fly in a determined country, it is required that the performance envelope and the TBO of the turbine engine(s) of the rotorcraft be certified by the official services of the country in question for a precise utilization spectrum. Such authorization can thus be obtained only after complete certification tests, and they are very expensive.
These complete certification tests of a turbine engine are performed in order to justify a performance envelope associated with a TBO, so, a priori, it is not possible to use that turbine engine with an alternative performance envelope, different from the performance envelope that was initially authorized, without performing new complete certification testing, which is very expensive.
It can happen, for example, that the above-described performance envelope associated with a TBO of about 2500 hours (h) corresponds to a utilization spectrum of a type that complies with most civilian applications. Nevertheless, for a military application or for certain special missions, e.g. a rescue mission requiring winching, that envelope can turn out to be unsuitable.